Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 1,853	$ 1,598	$ 6,183	$ 5,448
Interest cost	2,124	2,489	6,139	7,314
Expected return on plan assets	(6,386)	(6,099)	(19,196)	(18,249)
Amortization of prior service cost	(18)	(18)	(55)	(55)
Amortization of actuarial loss (gain)	1,313	230	2,923	360
Total net periodic pension cost	(1,114)	(1,800)	(4,006)	(5,183)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	20	26	59	78
Amortization of prior service cost	(19)	(19)	(56)	(56)
Amortization of actuarial loss (gain)	0	(1)	0	(3)
Total net periodic pension cost	$ 1	$ 6	$ 2	$ 19